netdev - Re: [PATCH net-next v2 1/1] ppp: Replace per-CPU recursion counter with lock-owner field

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Message-ID: <aHUsB04j+uFrUkpd@debian>
Date: Mon, 14 Jul 2025 18:10:47 +0200
From: Guillaume Nault <gnault@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: netdev@...r.kernel.org, linux-rt-devel@...ts.linux.dev,
	linux-ppp@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	Clark Williams <clrkwllms@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH net-next v2 1/1] ppp: Replace per-CPU recursion counter
 with lock-owner field

On Thu, Jul 10, 2025 at 06:24:03PM +0200, Sebastian Andrzej Siewior wrote:
> The per-CPU variable ppp::xmit_recursion is protecting against recursion
> due to wrong configuration of the ppp channels. The per-CPU variable

I'd rather say that it's the ppp unit that is badly configured: it's
the ppp unit that can creates the loop (as it creates a networking
interface).

> relies on disabled BH for its locking. Without per-CPU locking in
> local_bh_disable() on PREEMPT_RT this data structure requires explicit
> locking.
> 
> The ppp::xmit_recursion is used as a per-CPU boolean. The counter is
> checked early in the send routing and the transmit path is only entered
> if the counter is zero. Then the counter is incremented to avoid
> recursion. It used to detect recursion on channel::downl and
> ppp::wlock.
> 
> Create a struct ppp_xmit_recursion and move the counter into it.
> Add local_lock_t to the struct and use local_lock_nested_bh() for
> locking. Due to possible nesting, the lock cannot be acquired
> unconditionally but it requires an owner field to identify recursion
> before attempting to acquire the lock.
> 
> The counter is incremented and checked only after the lock is acquired.
> Since it functions as a boolean rather than a count, and its role is now
> superseded by the owner field, it can be safely removed.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  drivers/net/ppp/ppp_generic.c | 38 ++++++++++++++++++++++++++---------
>  1 file changed, 29 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
> index def84e87e05b2..0edc916e0a411 100644
> --- a/drivers/net/ppp/ppp_generic.c
> +++ b/drivers/net/ppp/ppp_generic.c
> @@ -119,6 +119,11 @@ struct ppp_link_stats {
>  	u64 tx_bytes;
>  };
>  
> +struct ppp_xmit_recursion {
> +	struct task_struct *owner;
> +	local_lock_t bh_lock;
> +};
> +

This hunk conflicts with latest changes in net-next.

Apart from the two minor comments above, the patch looks good to me.
Thanks!